It is common to store water in unpressurized above-ground vessels from which water from time to time is drawn by means of pump or gravity flow, particularly where the water supply is intermittent, or where the instantaneous demand exceeds the flow rate of the source of supply. It is particularly desirable, to have a vessel which is sealed against egress or ingress of liquids when potable water is being stored.
Typical water storage tanks that are associated with dwellings and other smaller buildings will have 500 to 1000 liter capacity. A squat cylindrical shape tank is popular for reason of compactness and manufacturability. A representative 500 liter tank might be about 100 cm in diameter and 100 cm high. One approach is to make a one piece plastic tank, such as by blow molding or rotational molding. Such tanks have good integrity but owing to their bulkiness, the cost of shipping from factory to point of use can be high.
The present invention is concerned with plastic tanks that principally are intended for use when oriented as vertical cylinders, including tanks which are much larger than just mentioned. For instance, plastic tanks may have capacities of in the range of about 9,500 liters to about 11,000 liters or more. A representative tank may have a diameter of about 8 feet (2.4 meters), a height of about 8 to 10 feet (2.4 to 3.1 meters), a basic wall thickness of 0.19 to 0.4 inches (4.8 to 10 mm) and a weight of about 250 to 400 pounds (114 to 180) kilograms. If such tanks can be fabricated by mating two bucket-like parts and which are then welded to each other at or near the point of use, that can be a practical solution when the parts have sufficient draft to enable nesting for shipment.
Tanks comprised of mating half tanks which can be nested for shipment and assembled in proximity to the point of use have been made. For example, see U.S. Pat. No. 5,878,907 of Graf and U.S. Pat. No. 8,740,005 of Holbrook et al. Such tanks have a mid-elevation joint which is alternatively secured by means of clamps or by means of welding.
Aims of the present invention relate to the fabrication such types of tanks and with verifying that the weld between flanges is good. When the weld joint is formed at a location which is not like the controlled environment of a manufacturing plant, perhaps by workers who infrequently do that kind of welding work, the joint should be able to readily made, and its integrity verified.
Richter U.S. Pat. No. 9,096,168 describes methodology that is related to the present invention. A tank is formed by joining two double-wall blow molded plastic segments to each other using inner and outer circumferential welds. A vacuum is drawn through a port drilled into one of the welds, to penetrate the annular space between the weld. If there is a leak, the vacuum will be lessened. See also Kops U.S. Pat. No. 4,002,055 for an application of the same technique for testing the joint between seams of joined-together sheet material.